Method for regeneration of an electrolysis bath for the production of a compound I-III-VI2 in thin layers

ABSTRACT

The invention relates to the regeneration of an electrolysis bath for the production of I-III-VI&lt;SB&gt;Y&lt;/SB&gt; compounds in thin layers, where y is approaching 2 and VI is an element including selenium, whereby selenium is regenerated in the form Se(IV) and/or with addition of oxygenated water to reoxidise the selenium in the bath to give the form Se(IV).

FIELD OF INVENTION

The present invention relates to the production of semiconductors of theI-III-VI₂ type in thin film form, especially for the design of solarcells.

BACKGROUND OF THE INVENTION

I-III-VI₂ compounds of the CuIn_(x)Ga_((1-x))Se_(y)S_((2-y)) type (wherex is substantially between 0 and 1 and y is substantially between 0 and2) are regarded as very promising and could constitute the nextgeneration of thin-film photovoltaic cells. These compounds have a widedirect bandgap of between 1.05 and 1.6 eV, which allows solar radiationin the visible to be strongly absorbed.

Record photovoltaic conversion efficiencies have been achieved bypreparing thin films by evaporation on small areas. However, evaporationis difficult to adapt to the industrial scale because of problems ofnonuniformity and low utilization of raw materials. Sputtering is bettersuited to large areas, but it requires very expensive vacuum equipmentand precursor targets.

There is therefore a real need for alternative, low-costatmospheric-pressure, techniques. The technique of thin-film depositionby electrochemistry, in particular by electrolysis, appears to be a veryattractive alternative. The advantages of this deposition technique arenumerous, and in particular the following:

-   -   deposition at ambient temperature and ambient pressure in an        electrolysis bath;    -   possibility of handling large areas with high uniformity;    -   ease of implementation;    -   low installation and raw material costs (no special forming        operation, high level of material utilization); and    -   great variety of possible deposit shapes due to the localized        nature of the deposit on the substrate.

Despite extensive research in this field, the difficulties encounteredrelate to the control of the quality of the electrodeposited precursors(composition and morphology) and the efficiency of the electrolysis bathafter several successive depositions.

OBJECTS OF THE INVENTION

It is an object of the present invention to propose a method ofproducing thin films of a I-III-VI_(y) compound (where y is close to 2)by electrolysis, which ensures that the deposition conditions are stableand reproducible.

A further object is to be able to carry out, over large areas, a largenumber of successive depositions of thin films having the desiredmorphology and the desired composition.

Another object of the present invention is to propose a method ofproducing thin films of the I-III-VI_(y) compound, which ensures asatisfactory lifetime of the electrolysis bath and effectiveregeneration of the raw materials consumed during the electrolysis.

Another object of the present invention is to propose a method ofproducing thin films of the I-III-VI_(Y) compound; which ensures thatthe raw materials consumed during the electrolysis are regenerated,without in any way causing the composition of the electrolysis bath togo out of equilibrium and therefore reducing its lifetime.

SUMMARY OF THE INVENTION

For this purpose, the subject of the invention is a method of producinga I-III-VI_(y) compound in thin film form by electrochemistry, in whichy is close to 2 and VI is an element comprising selenium, of the typecomprising the following steps:

-   -   a) of providing an electrolysis bath comprising active selenium,        in oxidation state IV, and at least two electrodes; and    -   b) of applying a potential difference between the two electrodes        in order to substantially promote migration of the active        selenium toward one of the electrodes and thus initiate the        formation of at least one thin film of I-III-VI_(Y).

Within the context of the invention, the method furthermore includes astep c) of regenerating the selenium in active form in said bath, inorder to increase the lifetime of said electrolysis bath.

Thus, within the context of the present invention, the method begins byregenerating the bath in terms of active selenium before regenerating itin terms of element I (such as copper) and/or element III (such asindium or gallium). This is because it has been found that a slightreintroduction of active selenium in the bath (preferably an excess ofabout 20% in molar concentration relative to the amount of seleniumnormally added) makes it possible again to obtain substantially the samenumber and the same volume of thin films as those obtained after stepb).

Advantageously, after step c), at least one new thin film ofI-III-VI_(Y) is formed.

Thus, in a first embodiment, at step c), selenium is added to the bathin order to form an excess of active selenium in the bath.

In another embodiment, as a variant of or in addition to theaforementioned first embodiment, at step c), an oxidizing agent forselenium is introduced into the bath in order to regenerate selenium inactive form.

Usually, the electrolysis bath, when it ages over the course of thedeposition, has selenium colloids. This selenium in colloid form is inoxidation state 0 and, within the context of the present invention, isnot capable of combining with the elements I and III.

Advantageously, if the bath contains selenium in colloid form at stepb), the aforementioned oxidizing agent is capable of regenerating theselenium in colloid form to selenium in active form.

Thus, it will be understood that the expression “selenium in activeform” means selenium in oxidation state IV, which is capable of beingreduced at the electrode to the ionic form SE²⁻ and of combiningnaturally with the elements I and III in order to form the thin films ofI-III-VI_(Y), and being distinguished from selenium in oxidation state0, for example in the form of colloids in the solution of the bath,which does not combine with the elements I and III.

In a particularly advantageous embodiment, said oxidizing agent ishydrogen peroxide, preferably with a concentration in the bath of theorder of magnitude corresponding substantially to at least five timesthe initial selenium concentration in the bath.

The addition of hydrogen peroxide to the bath therefore makes itpossible to regenerate the electrolysis bath at very low cost. Inaddition, this regeneration is carried out without contaminating thebath since a simple degassing operation allows the initial constitutionof the bath to be recovered.

In this regard, in which the electrolysis bath is regenerated bylimiting its contamination by the regenerating additives, it isadvantageous to provide a step after step c), of regenerating theelectrolysis bath by introducing oxides and/or hydroxides of elements Iand III.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features of the invention will become apparent onreading the detailed description below of embodiments given by way ofnonlimiting examples, and by examining the drawings which accompany it,in which:

FIG. 1 shows schematically a thin film obtained by implementing themethod according to the invention; and

FIG. 2 shows schematically an electrolysis bath for implementing themethod according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, copper indium diselenide films CO are obtained atroom pressure and room temperature by electrodeposition of a thinprecursor film of suitable composition and suitable morphology on aglass substrate S coated with molybdenum Mo. The term “precursor film”is understood to mean a thin layer of overall composition close toCuInSe₂ and obtained directly after deposition by electrolysis, withoutany subsequent treatment.

The electrodeposition is carried out using an acid bath B (FIG. 2),stirred by blades M, which contains an indium salt, a copper salt andselenium oxide in solution. The concentrations of these precursorelements are between 10⁻⁴ and 10⁻² M. The pH of the solution is setbetween 1 and 4.

Three electrodes, An, Ca and REF, including:

-   -   a molybdenum electrode Ca (standing for cathode) on which the        thin film forms by electrodeposition; and    -   a mercurous sulfate reference electrode REF, are immersed in the        bath B.

The electrical potential difference applied to the molybdenum electrodeis between −0.8 and −1.2 V relative to the reference electrode REF.

Layers having a thickness of between 1 and 4 microns are obtained withcurrent densities of between 0.5 and 10 mA/cm².

Under the defined composition, stirring and potential differenceconditions, it is possible to obtain dense adherent films of homogeneousmorphology, the composition of which is close to the stoichiometriccomposition: Cu (25%); In (25+ε %) and Se (50%), with a compositionslightly richer in indium, as Table I below shows. It is thus possibleto deposit films on areas of 10×10 cm².

An exemplary embodiment of the invention is given below.

A typical deposit was produced from a bath whose initial formulation wasthe following:

-   -   [CuSO₄]=1.0×10⁻³ M;    -   [In₂(SO₄)₃]=3.0×10⁻³ M;    -   [H₂SeO₃]=1.7×7.10⁻³ M;    -   [Na₂SO₄]=0.1 M,        where the notation “M” corresponds to the unit “mole per liter”,        for a pH of 2.2.

The precursors were deposited by a cathodic reaction for a set potentialof −1 V relative to the electrode REF. The current density was −1mA/cm².

After each electrolysis, the bath was recharged with elements Cu, In andSe on the basis of the number of coulombs indicated by a detection cell(not shown) which thus counts the number of ions that are interactedwith the solution of the bath. This recharging allowed the concentrationof the elements to be kept constant over the course of the successiveelectrodeposition operations. The pH could also be readjusted by addingsodium hydroxide (such as NaOH, for a concentration such as 1 M), butthis measure is not systematically necessary here, as will be seenlater.

Under these conditions, it was usually found that, after an indicationof 500±100 coulombs in a 1-liter solution (corresponding to theelectrodeposition of 4 to 5 thin films of 25 cm² area with a thicknessof 2 μm), partial or complete debonding of the CuInSe₂ filmssystematically occurs.

According to the invention, this debonding disappeared by regeneratingthe bath with selenium, before even regenerating the elements Cu and In.

A distinction should be made here between active selenium of oxidationstate IV, usually denoted Se(IV), and inactive selenium, in oxidationstate 0, which is generally observed in the form of colloids in theelectrolysis bath and usually denoted by Se(0).

It should be pointed out that it is only active selenium Se(IV) that iscapable of being reduced at the electrode Ca to the ionic form Se²⁻ andof being combined, in this form, with the elements Cu and In to form thethin films of CuInSe₂.

It should also be pointed out that there are two competitive reactionsduring the electrolysis: the selenium introduced into the bath can beconverted at the electrode:

-   -   either into Se²⁻ favorable to the formation of the thin films as        indicated above;    -   or to Se(0) in colloid form, which is not favorable to the        formation of thin films, especially because the colloids pose        problems at the interface between the substrate (or the        molybdenum layer MO here) and the thin Cu—In—Se film being        formed.

Advantageously, regeneration is carried out with an excess of Se(IV) inthe bath. For this purpose, selenium oxide is added, dissolved in theelectrolysis bath, in order to slow down the ageing of the bath. Inpractice, for a thin film formed and 115 coulombs passing through thesolution, it is theoretically necessary to add 1.8×10⁻⁴ M of [H₂SeO₃] tothe solution in order to have an initial selenium concentration of1.7×10⁻³ M again. An addition of twice this amount (i.e. 3.6×10⁻⁴ M andtherefore an excess of 1.8×10⁻⁴ M of [H₂SeO₃]), at the fifth deposition,makes it possible to obtain adherent films again. These thin films havethe desired morphology and the desired composition (Table I). Anover-regeneration of 3.6×10⁻⁴ M thus makes it possible to obtain a cycleof 4 to 5 films of satisfactory adhesion before further debondingproblems are observed. After each debonding cycle, the renewal of thisoperation allows adherent films to be obtained.

As a variant of or in addition to this operation, an oxidizing agent forreoxidizing the selenium in Se(0) form is used in order to obtainselenium in Se(IV) form. For this purpose, it is preferred to usehydrogen peroxide H₂O₂, employing H₂O₂ in large excess in the solution(concentration of the order of 10⁻² M, preferably close to 4×10⁻² M).The films become adherent again for 4 to 5 successive thin-filmdeposition operations, before they become debonded again. The renewal ofthis operation also makes it possible to obtain adherent films again.Advantageously, it has been observed that the addition of hydrogenperoxide furthermore makes it possible to obtain thin films ofrelatively smoother morphology.

Thus, it has been found that there is a great similarity between theeffects provided by Se(IV) over-regeneration and H₂O₂ addition to thesolution. It may also be pointed out that other types of oxidizing agentthan hydrogen peroxide, especially ozone O₃, may be used in order toincrease the lifetime of the baths.

The composition (Table I) and the morphology of the films aresubstantially the same as when hydrogen peroxide was added to the bathor when selenium (IV) was regenerated.

TABLE I Comparative analysis of the composition of the thinelectrodeposited CuInSe₂ films as a function of excess selenium Se(IV)over-regeneration and addition of hydrogen peroxide. Cu (%) In (%) Se(%) First deposit 21.4 27.5 51 Addition of H₂O₂ 22.9 25 52 Excessregeneration of 21.4 28.8 49.7 Se(IV)

The addition of hydrogen peroxide or the excess regeneration of Se(IV)makes it possible to considerably increase the number of films that canbe deposited with one bath. Such recycling of the bath makes it possiblefor the elements introduced, and more particularly the indium, to beentirely consumed by electrolysis. This makes it possible, particularlyadvantageously, to reduce the precursor production costs, especiallycompared with evaporation or sputtering methods.

It should be pointed out that, according to an advantageous aspect ofthe regeneration of the bath within the context of the invention, copperand/or indium oxides or hydroxides are also added in order to regeneratethe CuInSe₂ electrolysis bath in terms of copper and/or indium.

For example, by adding copper oxide CuO and indium oxide In₂O₃ to thebath, the following reactions (1) and (2) occur:CuO+H₂O→Cu²⁺+2OH⁻  (1)(½)In₂O₃+( 3/2)H₂O→In³⁺+3OH⁻  (2)

In contrast, if the compounds CuSO₄ and In₂(SO₄)₃ have been added, thebath would have been contaminated with SO₄ ²⁻ sulfate ions.

Furthermore, the reaction to form CuInSe₂ at the cathode is written as:Cu²⁺+In³⁺+2H₂SeO₃+8H⁺+13e⁻→CuInSe₂+6H₂O  (4)where e⁻ corresponds to an electron, whereas at the anode, the followingreaction takes place:(13/2)H₂O→13H⁺+(13/4)O₂+13e⁻  (4)in order to maintain charge equilibrium.

According to another advantage provided by the addition of Cu and Inoxides, it has been found that the difference of five H⁺ ions in excessbetween equations (3) and (4) is compensated for by the five OH⁻ ionsintroduced by the reactions (1) and (2). Thus it will be understood thatthe addition of Cu and In oxides furthermore makes it possible tostabilize the pH of the solution and to dispense with the addition ofsodium hydroxide as mentioned above.

It may furthermore be pointed out that the addition of hydroxidesCu(OH)₂ and In(OH)₃ produces the same effects, the reactions (1) and (2)becoming simply:Cu(OH)₂→Cu²⁺+2OH⁻  (1′)In(OH)₃In³⁺+3OH⁻  (2′)

Thus, the longevity and stability of the baths for electrodepositingI-III-VI_(y) compounds such as Cu—In—Se_(y) (with y close to 2) areensured by the addition of agents that do not affect the quality of thefilms. The electrodeposited precursor film contains the elements in acomposition close to I-III-VI₂ stoichiometry. The compositions and themorphology are controlled during the electrolysis. These agents (excessSe(IV) or H₂O₂) may be readily used for any type of electrolysis bathfor electrodepositing I-III-VI systems such as Cu—In—Ga—Al—Se—S.

The conversion efficiencies obtained (9% without a surfaceantireflection film) attest to the quality of the deposits obtained bythe method according to the invention.

Of course, the present invention is not limited to the embodimentdescribed above by way of example; rather it extends to otheralternative embodiments.

Thus, it will be understood that the elements I and III initiallyintroduced into the solution in CuSO₄ and In₂(SO₄)₃ form mayadvantageously be introduced rather in the form of copper and indiumoxides or hydroxides in order to limit contamination of the bath.

1. A method of producing a I-III-V_(y), compound in thin film form byelectrochemistry, in which y is close to 2, VI is an element comprisingselenium, I is copper, silver or gold and III is boron, aluminum,gallium, indium or thallium, comprising: a) providing an electrolysisbath comprising active selenium, in oxidation state IV (Se(IV)), and atleast two electrodes; b) applying a potential difference between the twoelectrodes to promote migration of the active selenium toward one of theelectrodes and initiate formation of at least one thin film of theI-III-VI_(y), compound; and, c) regenerating the selenium in active form(Se(IV)) in the electrolysis bath.
 2. The method of claim 1, wherein, atstep c), an oxidizing agent for selenium is introduced into theelectrolysis bath in order to regenerate the selenium in active form. 3.The method of claim 2, wherein when the electrolysis bath containsselenium in colloid form (Se(0)) at step b), the oxidizing agentregenerates the selenium in the colloid form to selenium in the activeform.
 4. The method of claim 2, wherein the oxidizing agent is hydrogenperoxide (H₂O₂).
 5. The method of claim 4, wherein the hydrogen peroxideis added to the electrolysis bath in a concentration at leastapproximately five times an initial selenium concentration in theelectrolysis bath.
 6. The method of claim 1, wherein, at step c),selenium is added to the electrolysis bath in order to form an excess ofactive selenium in the electrolysis bath.
 7. The method of claim 1,wherein, when one tenth of a concentration of selenium at step a) isconsumed by producing the thin film at step b), approximately twice theconsumed concentration of selenium is added to the bath at step c). 8.The method of claim 1, wherein, after step c), at least one new thinfilm of the I-III-VI_(y) compound is formed.
 9. The method of claim 1,wherein, the at least one thin film of the I-III-VI_(y) compound isCuInSe_(y) and the bath further comprises, at step a), for one unit ofconcentration of copper in the electrolysis bath, about 1.7 units ofconcentration of the active selenium.
 10. The method of claim 1, furthercomprising step d), regenerating the electrolysis bath by introducingoxides and/or hydroxides of elements I and III wherein, the oxide isIn₂O₃ and the hydroxide is In(OH)₃, or wherein the oxide is CuO and thehydroxide is Cu(OH)₂.